Switching power supply unit and electric power supply system

ABSTRACT

A switching power supply unit includes: a transformer; an inverter circuit including first to fourth switching devices, first to third capacitors, first and second rectifying devices, a resonant inductor, and a resonant capacitor; and a driver. The first to fourth switching devices are coupled in series. The first and second capacitors are coupled in series. The first rectifying device is disposed between a first connection point between the first and second capacitors and a second connection point between the first and second switching devices. The second rectifying device is disposed between the first connection point and a third connection point between the third and fourth switching devices. The third capacitor is disposed between the second and third connection points. The resonant capacitor, the resonant inductor, and a primary winding are coupled in series between a fourth connection point between the second and third switching devices and the first connection point.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Japanese Priority PatentApplication No. 2020-121514 filed on Jul. 15, 2020, the entire contentsof which are incorporated herein by reference.

BACKGROUND

The technology relates to a switching power supply unit that performsvoltage conversion using switching devices, and an electric power supplysystem including such a switching power supply unit.

As some examples of a switching power supply unit, various DC-DCconverters have been proposed and put into practical use (see, forexample, Japanese Unexamined Patent Application Publication No.2014-205190). Such DC-DC converters each typically include an invertercircuit, a power conversion transformer, and a rectifying and smoothingcircuit. The inverter circuit includes switching devices.

SUMMARY

A switching power supply unit according to one embodiment of thetechnology includes a pair of input terminals, a pair of outputterminals, a transformer, an inverter circuit, a rectifying andsmoothing circuit, and a driver. The pair of input terminals isconfigured to receive an input voltage. The pair of output terminals isconfigured to output an output voltage. The transformer includes aprimary winding and a secondary winding. The inverter circuit isdisposed between the pair of input terminals and the primary winding,and includes first to fourth switching devices, first to thirdcapacitors, first and second rectifying devices, a resonant inductor,and a resonant capacitor. The rectifying and smoothing circuit isdisposed between the pair of output terminals and the secondary winding,and includes a rectifying circuit and a smoothing circuit. Therectifying circuit includes two or more rectifying devices. Thesmoothing circuit includes a fourth capacitor. The driver is configuredto perform switching driving to control respective operations of thefirst to fourth switching devices in the inverter circuit. The first tofourth switching devices are coupled in series to each other in thisorder between two input terminals constituting the pair of inputterminals. The first capacitor and the second capacitor are coupled inseries to each other between the two input terminals constituting thepair of input terminals. The first rectifying device is disposed betweena first connection point and a second connection point. The firstconnection point is a connection point between the first capacitor andthe second capacitor. The second connection point is a connection pointbetween the first switching device and the second switching device. Thesecond rectifying device is disposed between the first connection pointand a third connection point. The third connection point is a connectionpoint between the third switching device and the fourth switchingdevice. The third capacitor is disposed between the second connectionpoint and the third connection point. The resonant capacitor, theresonant inductor, and the primary winding are coupled in series to eachother in no particular order between a fourth connection point and thefirst connection point. The fourth connection point is a connectionpoint between the second switching device and the third switchingdevice.

An electric power supply system according to one embodiment of thetechnology includes the switching power supply unit according to theembodiment of the technology, and a power source configured to supplythe input voltage to the pair of input terminals.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the disclosure and are incorporated in and constitute apart of this specification. The drawings illustrate example embodimentsand, together with the specification, serve to explain the principles ofthe technology.

FIG. 1 is a circuit diagram illustrating a schematic configurationexample of a switching power supply unit according to one exampleembodiment of the technology.

FIG. 2 is a timing waveform diagram illustrating an operation example ofthe switching power supply unit illustrated in FIG. 1 .

FIG. 3 is a circuit diagram illustrating an operation example at State Aillustrated in FIG. 2 .

FIG. 4 is a circuit diagram illustrating an operation example at State Billustrated in FIG. 2 .

FIG. 5 is a circuit diagram illustrating an operation example at State Cillustrated in FIG. 2 .

FIG. 6 is a circuit diagram illustrating an operation example at State Dillustrated in FIG. 2 .

FIG. 7 is a circuit diagram illustrating an operation example at State Eillustrated in FIG. 2 .

FIG. 8 is a circuit diagram illustrating an operation example at State Fillustrated in FIG. 2 .

FIG. 9 is a circuit diagram illustrating an operation example at State Gillustrated in FIG. 2 .

FIG. 10 is a circuit diagram illustrating an operation example at StateH illustrated in FIG. 2 .

FIG. 11 is a circuit diagram illustrating a schematic configurationexample of a switching power supply unit according to a comparativeexample.

FIG. 12 is a circuit diagram illustrating a schematic configurationexample of a switching power supply unit according to one modificationexample.

FIG. 13 is a circuit diagram illustrating a schematic configurationexample of a switching power supply unit according to one modificationexample.

DETAILED DESCRIPTION

In general, a reduction in power loss is demanded of a switching powersupply unit such as a DC-DC converter. It is desirable to provide aswitching power supply unit that makes it possible to reduce a powerloss, and an electric power supply system including such a switchingpower supply unit.

In the following, some example embodiments and modification examples ofthe technology are described in detail with reference to theaccompanying drawings. Note that the following description is directedto illustrative examples of the disclosure and not to be construed aslimiting the technology. Factors including, without limitation,numerical values, shapes, materials, components, positions of thecomponents, and how the components are coupled to each other areillustrative only and not to be construed as limiting the technology.Further, elements in the following example embodiments which are notrecited in a most-generic independent claim of the disclosure areoptional and may be provided on an as-needed basis. The drawings areschematic and are not intended to be drawn to scale. Like elements aredenoted with the same reference numerals to avoid redundantdescriptions. The description is given in the following order.

1. Example Embodiment (an example including a center-tap rectifyingcircuit)

2. Modification Examples

Modification Example 1 (an example including a bridge rectifyingcircuit)

Modification Examples 2 and 3 (examples in which the respectiverectifying circuits in the example embodiment and Modification Example 1are each replaced with a synchronous rectifying circuit)

3. Other Modification Examples

1. Example Embodiment

[Configuration]

FIG. 1 illustrates a schematic configuration example of a switchingpower supply unit according to an example embodiment of the technology,i.e., a switching power supply unit 1, in a circuit diagram. Theswitching power supply unit 1 may function as a DC-DC converter thatperforms voltage conversion on a direct-current input voltage Vinsupplied from a direct-current input power source 10 (e.g., a battery)into a direct-current output voltage Vout, and supplies electric powerto a load 9. Examples of the load 9 include an electronic apparatus anda battery. As described below, the switching power supply unit 1 may bea so-called “(insulated half-bridge) LLC resonant” DC-DC converter. Notethat the voltage conversion to be performed by the switching powersupply unit 1 may be either up-conversion (step-up) or down-conversion(step-down).

The direct-current input voltage Vin may correspond to a specific butnon-limiting example of an “input voltage” of one embodiment of thetechnology. The direct-current output voltage Vout may correspond to aspecific but non-limiting example of an “output voltage” of oneembodiment of the technology. The direct-current input power source 10may correspond to a specific but non-limiting example of a “powersource” of one embodiment of the technology. A system including thedirect-current input power source 10 and the switching power supply unit1 may correspond to a specific but non-limiting example of an “electricpower supply system” of one embodiment of the technology.

The switching power supply unit 1 includes two input terminals T1 andT2, two output terminals T3 and T4, an inverter circuit 2, a transformer3, a rectifying and smoothing circuit 4, and a driving circuit 5. Theswitching power supply unit 1 may further include an input smoothingcapacitor Cin. The direct-current input voltage Vin may be inputted tobetween the input terminals T1 and T2. The direct-current output voltageVout may be outputted from between the output terminals T3 and T4.

The input terminals T1 and T2 may correspond to a specific butnon-limiting example of a “pair of input terminals” of one embodiment ofthe technology. The output terminals T3 and T4 may correspond to aspecific but non-limiting example of a “pair of output terminals” of oneembodiment of the technology.

The input smoothing capacitor Cin may be disposed between a primaryhigh-voltage line L1H coupled to the input terminal T1 and a primarylow-voltage line L1L coupled to the input terminal T2. In a specific butnon-limiting example, at a location between the inverter circuit 2described below and the input terminals T1 and T2, a first end or oneend of the input smoothing capacitor Cin may be coupled to the primaryhigh-voltage line L1H, while a second end or another end of the inputsmoothing capacitor Cin may be coupled to the primary low-voltage lineL1L. The input smoothing capacitor Cin may be a capacitor adapted tosmooth the direct-current input voltage Vin inputted from the inputterminals T1 and T2.

[Inverter Circuit 2]

The inverter circuit 2 is disposed between the pair of input terminalsT1 and T2 and a primary winding 31 of the transformer 3 to be describedlater. The inverter circuit 2 includes four switching devices S1, S2,S3, and S4, capacitors C1 and C2, a capacitor Cf (a flying capacitor),rectifying diodes D1 and D2, a resonant inductor Lr, and a resonantcapacitor Cr. The inverter circuit 2 may thus be a so-called“half-bridge” and “neutral-point-clamped (NPC)” inverter circuit. Notethat the resonant inductor Lr may be configured by a leakage inductanceof the transformer 3 to be described later, or may be providedindependently of such a leakage inductance.

The switching device S1 may correspond to a specific but non-limitingexample of a “first switching device” of one embodiment of thetechnology. The switching device S2 may correspond to a specific butnon-limiting example of a “second switching device” of one embodiment ofthe technology. The switching device S3 may correspond to a specific butnon-limiting example of a “third switching device” of one embodiment ofthe technology. The switching device S4 may correspond to a specific butnon-limiting example of a “fourth switching device” of one embodiment ofthe technology. The capacitor C1 may correspond to a specific butnon-limiting example of a “first capacitor” of one embodiment of thetechnology. The capacitor C2 may correspond to a specific butnon-limiting example of a “second capacitor” of one embodiment of thetechnology. The capacitor Cf may correspond to a specific butnon-limiting example of a “third capacitor” of one embodiment of thetechnology. Further, the rectifying diode D1 may correspond to aspecific but non-limiting example of a “first rectifying device” of oneembodiment of the technology, and the rectifying diode D2 may correspondto a specific but non-limiting example of a “second rectifying device”of one embodiment of the technology.

For example, metal oxide semiconductor-field effect transistors(MOS-FETs), insulated gate bipolar transistors (IGBTs), or other switchdevices may be used as the switching devices S1 to S4. FIG. 1illustrates an example in which the switching devices S1 to S4 areconfigured by MOS-FETs. In the case where MOS-FETs are used as theswitching devices S1 to S4 like this example, parasitic capacitances andparasitic diodes of the MOS-FETs are usable to configure capacitors anddiodes (not illustrated in FIG. 1 ) to be coupled in parallel to theswitching devices S1 to S4.

In the inverter circuit 2, the capacitors C1 and C2 are coupled inseries to each other between the input terminals T1 and T2, i.e.,between the primary high-voltage line L1H and the primary low-voltageline L1L. In a specific but non-limiting example, the capacitor C1 maybe disposed between the primary high-voltage line L1H and a connectionpoint P1, and the capacitor C2 may be disposed between the connectionpoint P1 and the primary low-voltage line L1L. Further, between theinput terminals T1 and T2 described above, the four switching devicesS1, S2, S3, and S4 are coupled in series to each other in this order. Ina specific but non-limiting example, the switching device S1 may bedisposed between the primary high-voltage line L1H and a connectionpoint P2; the switching device S2 may be disposed between the connectionpoint P2 and a connection point P4; the switching device S3 may bedisposed between the connection point P4 and a connection point P3; andthe switching device S4 may be disposed between the connection point P3and the primary low-voltage line L1L.

Further, in the inverter circuit 2, the rectifying diode D1 is disposedbetween the connection point P1 and the connection point P2. Theconnection point P1 is a connection point between the capacitors C1 andC2. The connection point P2 is a connection point between the switchingdevices S1 and S2. In a specific but non-limiting example, in therectifying diode D1, an anode may be coupled to the connection point P1,and a cathode may be coupled to the connection point P2. Likewise, therectifying diode D2 is disposed between the connection point P1 and theconnection point P3. The connection point P3 is a connection pointbetween the switching devices S3 and S4. In a specific but non-limitingexample, in the rectifying diode D2, an anode may be coupled to theconnection point P3, and a cathode may be coupled to the connectionpoint P1, in contrast to the rectifying diode D1 described above.Further, the capacitor Cf is disposed between the connection point P2and the connection point P3, that is, between the cathode of therectifying diode D1 and the anode of the rectifying diode D2. In aspecific but non-limiting example, a first end of the capacitor Cf maybe coupled to the connection point P2, and a second end of the capacitorCf may be coupled to the connection point P3.

Further, the resonant capacitor Cr, the resonant inductor Lr, and theprimary winding 31 of the transformer 3 to be described later arecoupled in series to each other between the connection point P4 and theconnection point P1 described above. The connection point P4 is aconnection point between the switching devices S2 and S3. In a specificbut non-limiting example, as illustrated in FIG. 1 , a first end of theresonant capacitor Cr may be coupled to the connection point P4; asecond end of the resonant capacitor Cr may be coupled to a first end orone end of the resonant inductor Lr; a second end or another end of theresonant inductor Lr may be coupled to one end of the primary winding 31described above; and another end of the primary winding 31 may becoupled to the connection point P1.

The connection point P1 described above may correspond to a specific butnon-limiting example of a “first connection point” of one embodiment ofthe technology. The connection point P2 described above may correspondto a specific but non-limiting example of a “second connection point” ofone embodiment of the technology. The connection point P3 describedabove may correspond to a specific but non-limiting example of a “thirdconnection point” of one embodiment of the technology. The connectionpoint P4 described above may correspond to a specific but non-limitingexample of a “fourth connection point” of one embodiment of thetechnology.

With such a configuration, in the inverter circuit 2, the switchingdevices S1 to S4 may perform switching operations, i.e., on and offoperations in accordance with drive signals SG1 to SG4 supplied from thedriving circuit 5 to be described later. This allows the direct-currentinput voltage Vin applied to between the input terminals T1 and T2 to beconverted into an alternating-current voltage (a voltage Vp), and allowsthe resulting alternating-current voltage to be outputted to thetransformer 3 (the primary winding 31).

[Transformer 3]

The transformer 3 may include the primary winding 31 and two secondarywindings 321 and 322.

The primary winding 31 may have a first end (the one end) coupled to thesecond end (the other end) of the resonant inductor Lr described above,and a second end (the other end) coupled to the connection point P1described above.

The secondary winding 321 may have a first end coupled to a cathode of arectifying diode 41 to be described later via a connection line L21 tobe described later. The secondary winding 321 may have a second endcoupled to a center tap P6 in the rectifying and smoothing circuit 4 tobe described later. The secondary winding 322 may have a first endcoupled to a cathode of a rectifying diode 42 to be described later viaa connection line L22 to be described later. The secondary winding 322may have a second end coupled to the center tap P6 described above. Thatis, the second end of the secondary winding 321 and the second end ofthe secondary winding 322 may be coupled in common to the center tap P6.

The transformer 3 may be configured to perform voltage conversion on avoltage generated by the inverter circuit 2, that is, the voltage Vp(see FIG. 1 ) in the form of a rectangular pulse wave to be inputted tothe primary winding 31 of the transformer 3, and to output analternating-current voltage, i.e., a voltage Vs, from the end of each ofthe secondary windings 321 and 322. In a specific but non-limitingexample, a voltage Vs1 may be outputted from the secondary winding 321,and a voltage Vs2 may be outputted from the secondary winding 322 (seeFIG. 1 ). Note that the voltage conversion degree of the direct-currentoutput voltage Vout with respect to the direct-current input voltage Vinin this case may be determined on the basis of a turns ratio of theprimary winding 31 to the secondary windings 321 and 322, and a dutyratio of each of ON periods Ton1 and Ton2 (see FIG. 2 ) to a switchingcycle Tsw to be described later.

[Rectifying and Smoothing Circuit 4]

The rectifying and smoothing circuit 4 may include two rectifying diodes41 and 42 and a single output smoothing capacitor Cout. In a specificbut non-limiting example, the rectifying and smoothing circuit 4includes a rectifying circuit including the rectifying diodes 41 and 42,and a smoothing circuit including the output smoothing capacitor Cout.

The two rectifying diodes 41 and 42 described above may correspond to aspecific but non-limiting example of “two or more rectifying devices” ofone embodiment of the technology. The output smoothing capacitor Coutmay correspond to a specific but non-limiting example of a “fourthcapacitor” of one embodiment of the technology.

The rectifying circuit described above may be a so-called “center-tap”rectifying circuit. That is, respective anodes of the rectifying diodes41 and 42 may be coupled to a ground line LG; the cathode of therectifying diode 41 may be coupled to the above-described first end ofthe secondary winding 321 via the connection line L21; and the cathodeof the rectifying diode 42 may be coupled to the above-described firstend of the secondary winding 322 via the connection line L22. Further,as described above, the respective second ends of the secondary windings321 and 322 may be coupled in common to the center tap P6. The centertap P6 may be coupled to the output terminal T3 described above via anoutput line LO. Note that the ground line LG described above may becoupled to the output terminal T4 described above.

In the smoothing circuit described above, the output smoothing capacitorCout may be coupled between the output line LO described above and theground line LG, i.e., between the output terminals T3 and T4. That is, afirst end of the output smoothing capacitor Cout may be coupled to theoutput line LO, and a second end of the output smoothing capacitor Coutmay be coupled to the ground line LG.

In the rectifying and smoothing circuit 4 having such a configuration,the rectifying circuit including the rectifying diodes 41 and 42 mayrectify the alternating-current voltage (the voltage Vs) outputted fromthe transformer 3, and then output the rectified voltage. Further, thesmoothing circuit including the output smoothing capacitor Cout maysmooth the voltage rectified by the rectifying circuit described aboveto generate the direct-current output voltage Vout. The direct-currentoutput voltage Vout thus generated may allow electric power to besupplied to the load 9 described above from the output terminals T3 andT4.

[Driving Circuit 5]

The driving circuit 5 is a circuit that performs switching driving tocontrol the respective operations of the switching devices S1 to S4 inthe inverter circuit 2. In a specific but non-limiting example, thedriving circuit 5 may be configured to supply the switching devices S1to S4 with the respective drive signals SG1 to SG4 independently of eachother to thereby control the respective switching operations, i.e., onand off operations, of the switching devices S1 to S4.

In controlling the switching operations, i.e., performing the switchingdriving, of the switching devices S1 to S4, the driving circuit 5 mayperform pulse width control, which will be described in detail later.That is, the driving circuit 5 may perform pulse width modulation (PWM)control on the drive signals SG1 to SG4.

Further, while a detailed description will be given later, the drivingcircuit 5 may perform the above-described switching driving in such amanner that respective switching frequencies fsw of the switchingdevices S1 to S4 are identical or substantially identical with eachother and constant or substantially constant.

Note that the driving circuit 5 described above may correspond to aspecific but non-limiting example of a “driver” of one embodiment of thetechnology.

Operations, Workings, and Effects A. Basic Operation

In the switching power supply unit 1, the direct-current input voltageVin supplied from the direct-current input power source 10 via the inputterminals T1 and T2 may be switched by the inverter circuit 2 togenerate a voltage in the form of a rectangular pulse wave, i.e., thevoltage Vp. The voltage in the form of a rectangular pulse wave may besupplied to the primary winding 31 of the transformer 3, and may then betransformed by the transformer 3. The transformed alternating-currentvoltage, i.e., the voltage Vs, may thus be outputted from each of thesecondary windings 321 and 322.

In the rectifying and smoothing circuit 4, the alternating-currentvoltage outputted from the transformer 3, i.e., the transformedalternating-current voltage described above, may be rectified by therectifying diodes 41 and 42 in the rectifying circuit and then smoothedby the output smoothing capacitor Cout in the smoothing circuit. Thedirect-current output voltage Vout may be thereby outputted from theoutput terminals T3 and T4. The direct-current output voltage Vout maythen allow electric power to be supplied to the load 9.

B. Detailed Operation

Next, a description will be given of detailed operation of the switchingpower supply unit 1, i.e., details of the pulse width control describedabove, with reference to FIGS. 2 to 10 as well as FIG. 1 .

FIG. 2 illustrates an operation example of the switching power supplyunit 1 in a timing waveform diagram. Specifically, parts (A) to (D) ofFIG. 2 illustrate voltage waveforms of the drive signals SG1 to SG4described above, respectively, and parts (E) and (F) of FIG. 2illustrate voltage waveforms of the voltages Vp and Vs described above,respectively. The horizontal axis of FIG. 2 represents time t.

Note that a period during which each of the drive signals SG1 to SG4 is“high (H)” may correspond to a period during which a corresponding oneof the switching devices S1 to S4 is on. A period during which each ofthe drive signals SG1 to SG4 is “low (L)” may correspond to a periodduring which a corresponding one of the switching devices S1 to S4 isoff.

Further, in the example of FIG. 2 , phase differences ϕ (i.e., a phasedifference ϕ1 between the drive signals SG1 and SG2 and a phasedifference ϕ2 between the drive signals SG3 and SG4) upon the pulsewidth control by a phase shift method are indicated, which will bedescribed in detail later. Note that ϕ1≈ϕ2. Further, in the example ofFIG. 2 , a period during which both the drive signals SG1 and SG4 are“L”, that is, a period during which both the switching devices S1 and S4are off, which will be referred to as a first dead time, is indicated asa dead time Td14. Likewise, in the example of FIG. 2 , a period duringwhich both the drive signals SG2 and SG3 are “L”, that is, a periodduring which both the switching devices S2 and S3 are off, which will bereferred to as a second dead time, is indicated as a dead time Td23. Inaddition, in the example of FIG. 2 , a period during which both thedrive signals SG1 and SG2 are “H”, that is, a period during which boththe switching devices S1 and S2 are on, which will be referred to as afirst ON period, is indicated as an ON period Ton1. Likewise, in theexample of FIG. 2 , a period during which both the drive signals SG3 andSG4 are “H”, that is, a period during which both the switching devicesS3 and S4 are on, which will be referred to as a second ON period, isalso indicated as an ON period Ton2.

Further, in FIG. 2 , eight states (respective states indicated as “StateA” to “State H” in FIG. 2 ) are set along the time t. These eight statesmay be sequentially repeated (i.e., in the order from “State A” to“State H”) to define a switching cycle Tsw (=1/fsw) and a switchingfrequency fsw (see FIG. 2 ). In a specific but non-limiting example, theperiod from a timing t1 to a timing t2 illustrated in FIG. 2 , forexample, may correspond to the switching cycle Tsw described above. Theswitching cycle Tsw may include the ON periods Ton1 and Ton 2, i.e., thefirst ON period and the second ON period described above, and the deadtimes Td14 and Td23, i.e., the first dead time and the second dead timedescribed above (see FIG. 2 ) FIGS. 3 to 10 illustrate respectiveoperation examples at the above-described eight states illustrated inFIG. 2 (the states indicated as “State A” to “State H” as describedabove) in circuit diagrams. In the following, the respective operationexamples at these states will be described in detail with reference toFIG. 2 . Note that regarding the switching devices S1 to S4 configuredby MOS-FETs as described above, parasitic diodes Dp2 and Dp3 of theswitching devices S2 and S3 and parasitic capacitances Cp1 and Cp4 ofthe switching devices S1 and S4 are each illustrated as appropriate inFIGS. 3 to 10 .

[State A]

First, at the “State A” illustrated in FIG. 3 , the switching devices S1and S2 may each be set at an ON state, whereas the switching devices S3and S4 may each be set at an OFF state (see parts (A) to (D) of FIG. 2). Then, on the primary side of the transformer 3, a primary circuitcurrent (a current Ip) flows from the direct-current input power source10 through the primary high-voltage line L1H, the switching device S1,the switching device S2, the resonant capacitor Cr, the resonantinductor Lr, the primary winding 31, the capacitor C2, and the primarylow-voltage line L1L in this order and back to the direct-current inputpower source 10. At this time, the value of the voltage Vp is asfollows: Vp=(Vin/2) (see part (E) of FIG. 2 ). Then, on the secondaryside of the transformer 3, a secondary circuit current (a current Is)flows from the secondary winding 322 through the output line LO, theoutput smoothing capacitor Cout and the load 9, the ground line LG, andthe rectifying diode 42 in this order and back to the secondary winding322.

[State B]

Next, at the “State B” illustrated in FIG. 4 , the switching device S1may be turned off (see part (A) of FIG. 2 ). Then, on the primary sideof the transformer 3, the primary circuit current (the current Ip) flowsfrom the resonant inductor Lr through the primary winding 31, therectifying diode D1, the switching device S2, and the resonant capacitorCr in this order and back to the resonant inductor Lr. At this time,electric charge is discharged from the parasitic capacitance Cp4 of theswitching device S4. The value of the voltage Vp at this time is asfollows: Vp=0 (see part (E) of FIG. 2 ). Then, on the secondary side ofthe transformer 3, the secondary circuit current (the current Is) flowsfrom the secondary winding 322 through the output line LO, the outputsmoothing capacitor Cout and the load 9, the ground line LG, and therectifying diode 42 in this order and back to the secondary winding 322.

[State C]

Next, at the “State C” illustrated in FIG. 5 , the switching device S4may be turned on (see part (D) of FIG. 2 ) and zero-voltage switching(ZVS) may be performed. Then, on the primary side of the transformer 3,the primary circuit current (the current Ip) flows from the resonantinductor Lr through the primary winding 31, the rectifying diode D1, theswitching device S2, and the resonant capacitor Cr in this order andback to the resonant inductor Lr. At this time, the value of the voltageVp is as follows: Vp=0 (see part (E) of FIG. 2 ). Then, on the secondaryside on the transformer 3, the secondary circuit current (the currentIs) flows from the secondary winding 322 through the output line LO, theoutput smoothing capacitor Cout and the load 9, the ground line LG, andthe rectifying diode 42 in this order and back to the secondary winding322.

[State D]

Next, at the “State D” illustrated in FIG. 6 , the switching device S2may be turned off (see part (B) of FIG. 2 ). Then, on the primary sideof the transformer 3, the primary circuit current (the current Ip) flowsfrom the resonant inductor Lr through the resonant capacitor Cr, theparasitic diode Dp2 of the switching device S2, the capacitor Cf, theswitching device S4, the primary low-voltage line L1L, the capacitor C2,and the primary winding 31 in this order and back to the resonantinductor Lr. Thus, the direction of flow of the current Ip during thepreceding period from the “State A” to the “State C” is reversed. Atthis time, the parasitic diode Dp2 described above brings the voltageinto a clamped state. The value of the voltage Vp at this time is asfollows: Vp=0 (see part (E) of FIG. 2 ). Then, on the secondary side ofthe transformer 3, the secondary circuit current (the current Is) flowsfrom the secondary winding 321 through the output line LO, the outputsmoothing capacitor Cout and the load 9, the ground line LG, and therectifying diode 41 in this order and back to the secondary winding 321.Thus, the current Is flows through a path different from that during thepreceding period from the “State A” to the “State C”.

[State E]

Next, at the “State E” illustrated in FIG. 7 , the switching device S3may be turned on (see part (C) of FIG. 2 ). Then, on the primary side ofthe transformer 3, the primary circuit current (the current Ip) flowsfrom the direct-current input power source 10 through the primaryhigh-voltage line L1H, the capacitor C1, the primary winding 31, theresonant inductor Lr, the resonant capacitor Cr, the switching deviceS3, the switching device S4, and the primary low-voltage line L1L inthis order and back to the direct-current input power source 10. Thevalue of the voltage Vp at this time is as follows: Vp=−(Vin/2) (seepart (E) of FIG. 2 ). Then, on the secondary side of the transformer 3,the secondary circuit current (the current Is) flows from the secondarywinding 321 through the output line LO, the output smoothing capacitorCout and the load 9, the ground line LG, and the rectifying diode 41 inthis order and back to the secondary winding 321.

[State F]

Next, at the “State F” illustrated in FIG. 8 , the switching device S4may be turned off (see part (D) of FIG. 2 ). Then, on the primary sideof the transformer 3, the primary circuit current (the current Ip) flowsfrom the resonant inductor Lr through the resonant capacitor Cr, theswitching device S3, the rectifying diode D2, and the primary winding 31in this order and back to the resonant inductor Lr. At this time,electric charge is discharged from the parasitic capacitance Cp1 of theswitching device S1. The value of the voltage Vp at this time is asfollows: Vp=0 (see part (E) of FIG. 2 ). Then, on the secondary side ofthe transformer 3, the secondary circuit current (the current Is) flowsfrom the secondary winding 321 through the output line LO, the outputsmoothing capacitor Cout and the load 9, the ground line LG, and therectifying diode 41 in this order and back to the secondary winding 321.

[State G]

Next, at the “State G” illustrated in FIG. 9 , the switching device S1may be turned on (see part (A) of FIG. 2 ) and ZVS may be performed.Then, on the primary side of the transformer 3, the primary circuitcurrent (the current Ip) flows from the resonant inductor Lr through theresonant capacitor Cr, the switching device S3, the rectifying diode D2,and the primary winding 31 in this order and back to the resonantinductor Lr. The value of the voltage Vp at this time is as follows:Vp=0 (see part (E) of FIG. 2 ). Then, on the secondary side of thetransformer 3, the secondary circuit current (the current Is) flows fromthe secondary winding 321 through the output line LO, the outputsmoothing capacitor Cout and the load 9, the ground line LG, and therectifying diode 41 in this order and back to the secondary winding 321.

[State H]

Next, at the “State H” illustrated in FIG. 10 , the switching device S3may be turned off (see part (C) of FIG. 2 ). Then, on the primary sideof the transformer 3, the primary circuit current (the current Ip) flowsfrom the resonant inductor Lr through the primary winding 31, thecapacitor C1, the primary high-voltage line L1H, the switching deviceS1, the capacitor Cf, the parasitic diode Dp3 of the switching deviceS3, and the resonant capacitor Cr in this order and back to the resonantinductor Lr. Thus, the direction of flow of the current Ip during thepreceding period from the “State E” to the “State G” is reversed. Atthis time, the parasitic diode Dp3 described above brings the voltageinto a clamped state. The value of the voltage Vp at this time is asfollows: Vp=0 (see part (E) of FIG. 2 ). Then, on the secondary side ofthe transformer 3, the secondary circuit current (the current Is) flowsfrom the secondary winding 322 through the output line LO, the outputsmoothing capacitor Cout and the load 9, the ground line LG, and therectifying diode 42 in this order and back to the secondary winding 322.Thus, the current Is flows through a path different from that during thepreceding period from the “State E” to the “State G”.

Here, as indicated with arrows in parts (A) to (D) of FIG. 2 , thedriving circuit 5 may adjust a start timing or a stop timing of each ofthe respective periods during which the switching devices S1 to S4 areon (i.e., the “H”-state periods). In a specific but non-limitingexample, the driving circuit 5 may adjust the start timing of each ofthe respective periods during which the switching devices S2 and S3 areon, that is, the timing of the rise of each of the drive signals SG2 andSG3. Further, the driving circuit 5 may adjust the stop timing of eachof the respective periods during which the switching devices S1 and S4are on, that is, the timing of the fall of each of the drive signals SG1and SG4. It is to be noted that in a case where the timing of the riseof each of the drive signals SG2 and SG3 is adjusted, the switchingcycle Tsw described above is defined by a period from the timing of therise of the drive signal SG1 to the timing of the rise of the drivesignal SG1 at a next cycle, or by a period from the timing of the riseof the drive signal SG4 to the timing of the rise of the drive signalSG4 at a next cycle. By adjusting the start timing or the stop timing ofeach of such respective ON-state periods, the driving circuit 5 maycontrol the value of the direct-current output voltage Vout as indicatedwith arrows in part (F) of FIG. 2 , for example. Note that theillustration of the direct-current output voltage Vout in FIG. 2disregards any voltage drops at the rectifying diodes 41 and 42.

Further, as illustrated in FIG. 2 , for example, the driving circuit 5may perform adjustments so that the ON period Ton1 during which both theswitching devices S1 and S2 are on and the ON period Ton2 during whichboth the switching devices S3 and S4 are to be on do not overlap eachother in the switching cycle Tsw. Then, the driving circuit 5 mayadjust, for example, respective duty ratios of these ON periods Ton1 andTon2 to the switching cycle Tsw to thereby control the value of thedirect-current output voltage Vout as indicated with the arrows in part(F) of FIG. 2 . That is, in the example of part (E) of FIG. 2 , a dutyratio DR1 of the ON period Ton1 (=Ton1/Tsw) during which Vp=(Vin/2) anda duty ratio DR2 of the ON period Ton2 (=Ton2/Tsw) during whichVp=−(Vin/2) may each be adjusted to thereby control the value of thedirect-current output voltage Vout.

Further, the driving circuit 5 may adjust, for example, the respectiveswitching frequencies fsw of the switching devices S1 to S4 (theswitching cycle Tsw) and the ON periods Ton1 and Ton2 described above tothereby control the value of the direct-current output voltage Vout asindicated with the arrows in part (F) of FIG. 2 . That is, the drivingcircuit 5 may perform hybrid control combining the pulse widthmodulation (PWM) control described above where the respective switchingfrequencies fsw are constant and pulse frequency modulation (PFM)control that adjusts the values of the respective switching frequenciesfsw.

The description of a series of operations, i.e., the pulse width controlin the switching cycle Tsw, with reference to FIG. 2 and FIGS. 3 to 10thus ends.

C. Workings and Effects

Next, example workings and example effects of the switching power supplyunit 1 of the present example embodiment will be described in detail incomparison with a comparative example.

C-1. Comparative Example

FIG. 11 illustrates a schematic configuration example of a switchingpower supply unit according to a comparative example, i.e., a switchingpower supply unit 101, in a circuit diagram. The switching power supplyunit 101 of this comparative example is an existing typical “LLCresonant” DC-DC converter. Specifically, the switching power supply unit101 corresponds to the switching power supply unit 1 of the presentexample embodiment illustrated in FIG. 1 in which the inverter circuit2, the transformer 3, and the driving circuit 5 are replaced with aninverter circuit 102, a transformer 103, and a driving circuit 105,respectively.

The inverter circuit 102 corresponds to the inverter circuit 2 of thepresent example embodiment with the following changes. Firstly, thecapacitors C1, C2, and Cf and the rectifying diodes D1 and D2 areomitted, and further, two switching devices S1 and S2 coupled in seriesto each other are provided in place of the four switching devices S1 toS4 coupled in series to each other. Specifically, the switching deviceS1 is disposed between the primary high-voltage line L1H and theconnection point P4, and the switching device S2 is disposed between theconnection point P4 and the primary low-voltage line L1L. Furthermore,the resonant capacitor Cr, the resonant inductor Lr, and the primarywinding 31 of the transformer 103 are coupled in series to each otherbetween the connection point P4 described above and the primarylow-voltage line L1L, in contrast to the case of the inverter circuit 2and the transformer 3 described previously.

The driving circuit 105 is a circuit that performs switching driving tocontrol respective operations of the switching devices S1 and S2 in theinverter circuit 102. Specifically, the driving circuit 105 isconfigured to supply the switching devices S1 and S2 with respectivedrive signals SG1 and SG2 independently of each other to thereby controlthe respective switching operations, i.e., on and off operations, of theswitching devices S1 and S2.

In the switching power supply unit 101 of the comparative example (i.e.,a typical “LLC resonant” DC-DC converter) having such a configuration,it is necessary to control the switching frequency fsw in order tostabilize the direct-current output voltage Vout. This makes theswitching devices S1 and S2 narrower in operation range where softswitching is possible. Accordingly, if the direct-current input voltageVin or the direct-current output voltage Vout varies greatly, avariation range of the switching frequency fsw becomes wider. This cangive rise to concerns as described below in a case where, as in thiscomparative example, the ratio of the direct-current output voltage Voutto the direct-current input voltage Vin, i.e., the output to inputvoltage ratio, is high and the operating voltage range is wide.

Firstly, in the switching power supply unit 101 of the comparativeexample, the transformer 103 has a high transformation ratio or turnsratio (e.g., about 16:1), and therefore the primary winding 31 has alarge number of turns in the transformer 103. This can lead to a largeloss in the primary winding 31. Thus, in the switching power supply unit101 of the comparative example, a large power loss can result due to thelarge loss in the primary winding 31 of the transformer 103.

It is to be noted that, in the switching power supply unit 101 of thecomparative example, the switching frequency fsw varies over a widerange (e.g., from about 800 kHz to about 2 MHz) and it is thereforedifficult to perform soft switching for each of the switching devices S1and S2. This can result in an increase in switching loss and can thusnecessitate an increase in size of a component such as a heatdissipation component. Thus, in the switching power supply unit 101 ofthe comparative example, a component such as the heat dissipationcomponent can be increased in size and accordingly, the entire switchingpower supply unit 101 can be increased in size. In addition, in theswitching power supply unit 101 of the comparative example, burstcontrol is needed when the load 9 is light or zero, for example.

C-2. Present Example Embodiment

In contrast, the switching power supply unit 1 of the present exampleembodiment is able to provide, for example, the following workings andeffects in contrast to the switching power supply unit 101 of thecomparative example described above, for example.

Firstly, according to the present example embodiment, owing to theinverter circuit 2 having the above-described circuit configurationincluding the capacitors C1, C2, and Cf, the rectifying diodes D1 andD2, etc., the voltage Vp, i.e., the voltage to be applied to theresonant circuit including the resonant inductor Lr and the resonantcapacitor Cr is formed into a rectangular pulse wave (see part (E) ofFIG. 2 ). This allows an amplitude of the fundamental wave of thevoltage Vp (−(4/π)×(Vin/2)×sin(D2×π) to +(4/π)×(Vin/2)×sin(D1×π)) to bemade smaller by adjusting the duty ratio, thus making it possible to setthe transformation ratio of the transformer 3 to a low value. Forexample, the present example embodiment makes it possible to set thetransformation ratio to 8:1, which is half the value of thetransformation ratio in the comparative example, i.e., 16:1. As aresult, in the transformer 3 of the present example embodiment, it ispossible to reduce the number of turns of the primary winding 31 to, forexample, ½ that in the transformer 103 of the above-describedcomparative example, and it is therefore possible to reduce a loss inthe primary winding 31.

Consequently, according to the present example embodiment, it ispossible to reduce a power loss in the switching power supply unit 1 ascompared with, for example, the above-described comparative example.

Further, in the present example embodiment, as has been described, theswitching driving of the switching devices S1 to S4 may be performed insuch a manner that the respective switching frequencies fsw of theswitching devices S1 to S4 are identical with each other and constant.This results in the following. That is, soft switching of each of theswitching devices S1 to S4 is facilitated as compared with the case ofthe above-described comparative example, and therefore switching loss isreduced as compared with the case of the above-described comparativeexample. As a result, it is possible to achieve a reduction in size of acomponent such as a heat dissipation component. Accordingly, the presentexample embodiment makes it possible to achieve a reduction in size ofthe switching power supply unit 1 as compared with, for example, theabove-described comparative example.

Further, in the present example embodiment, it is possible to controlthe value of the direct-current output voltage Vout in theabove-described manner. Furthermore, it is possible for the rectifyingdiodes 41 and 42 to operate in a discontinuous mode. It is thus possibleto achieve a reduction in noise. In addition, it is possible to improvethe reliability of the switching power supply unit 1.

Further, according to the present example embodiment, the value of thedirect-current output voltage Vout may be controlled by shifting thephases of the switching devices S1 to S4 (the drive signals SG1 to SG4)from each other while fixing the respective duty ratios of the switchingdevices S1 to S4. This makes it possible to easily control the value ofthe direct-current output voltage Vout. Further, this simplifies thecontrol (i.e., switching driving) of the switching devices S1 to S4,thus making it possible to improve the reliability of the switchingpower supply unit 1. It is to be noted that in a case where the ONperiod of each switching device Sn (n=1 to 4) is represented as Ton (n)(here, Ton (1)≈Ton (2)≈Ton (3)≈Ton (4)), the duty ratio of eachswitching device Sn described above is represented as (Ton (n)/Tsw),using the switching cycle Tsw.

Further, in the present example embodiment, the resonant inductor Lr inthe inverter circuit 2 may be configured by the leakage inductance ofthe transformer 3. This makes it unnecessary to separately provide theresonant inductor Lr, thus allowing for a reduction in the number ofcomponents. As a result, it is possible for the switching power supplyunit 1 to achieve a further reduction in size and a reduction in cost.

In addition, according to the present example embodiment, each of theswitching devices S1 to S4 in the inverter circuit 2 may be configuredby a MOS-FET. This makes it possible to raise the switching frequencyfsw, thus making it possible to achieve a reduction in component size.

Further, according to the present example embodiment, the rectifyingcircuit in the rectifying and smoothing circuit 4 may be a center-taprectifying circuit. This allows the number of the rectifying devices tobe reduced to two (the rectifying diodes 41 and 42) as compared withModification Example 1 to be described below, for example. As a result,it is possible to achieve reductions in size, loss, and cost of therectifying circuit.

2. Modification Examples

Next, modification examples (Modification Examples 1 to 3) of theforegoing example embodiment will be described. It is to be noted that,in the following description, components substantially the same as thoseof the switching electric power supply system 1 according to theforegoing example embodiment are denoted by the same reference signs,and descriptions thereof are omitted as appropriate.

Modification Example 1

[Configuration]

FIG. 12 illustrates a schematic configuration example of a switchingpower supply unit according to Modification Example 1, i.e., a switchingpower supply unit 1A, in a circuit diagram.

As with the foregoing example embodiment, a system including thedirect-current input power source 10 and the switching power supply unit1A may correspond to a specific but non-limiting example of the“electric power supply system” of one embodiment of the technology.

The switching power supply unit 1A of Modification Example 1 correspondsto the power supply unit 1 of the foregoing example embodiment in whichthe transformer 3 and the rectifying and smoothing circuit 4 arereplaced with a transformer 3A and a rectifying and smoothing circuit4A, respectively. The remainder of configuration of the switching powersupply unit 1A may be similar to that of the switching power supply unit1.

The transformer 3A may include a single primary winding 31 and a singlesecondary winding 32. That is, in contrast to the transformer 3including the two secondary windings 321 and 322, the transformer 3A mayinclude only a single secondary winding 32. The secondary winding 32 mayhave a first end coupled to a connection point P7 in the rectifying andsmoothing circuit 4A to be described later, and a second end coupled toa connection point P8 in the rectifying and smoothing circuit 4A.

Like the transformer 3, the transformer 3A may be configured to performvoltage conversion on a voltage generated by the inverter circuit 2,that is, the voltage Vp in the form of a rectangular pulse wave, and tooutput an alternating-current voltage, i.e., the voltage Vs, from theend of the secondary winding 32. Note that the voltage conversion degreeof the output voltage with respect to the input voltage in this case maybe determined on the basis of the turns ratio of the primary winding 31to the secondary winding 32, and the duty ratios of the ON periods Ton1and Ton2 to the foregoing switching cycle Tsw.

The rectifying and smoothing circuit 4A may include four rectifyingdiodes 41 to 44 and a single output smoothing capacitor Cout. In aspecific but non-limiting example, the rectifying and smoothing circuit4A includes a rectifying circuit including the rectifying diodes 41 to44, and a smoothing circuit including the output smoothing capacitorCout. That is, the rectifying and smoothing circuit 4A may correspond tothe rectifying and smoothing circuit 4 with its configuration modified.

Note that the four rectifying diodes 41 to 44 described above maycorrespond to a specific but non-limiting example of the “two or morerectifying devices” of one embodiment of the technology.

The rectifying circuit of Modification Example 1 may be a so-called“bridge” rectifying circuit, being different from the “center-tap”rectifying circuit of the foregoing example embodiment. That is,respective cathodes of the rectifying diodes 41 and 43 may be coupled tothe output line LO; and the anode of the rectifying diode 41 may becoupled to the cathode of the rectifying diode 42 and theabove-described first end of the secondary winding 32 at the connectionpoint P7. Further, respective anodes of the rectifying diodes 42 and 44may be coupled to the ground line LG; and a cathode of the rectifyingdiode 44 may be coupled to an anode of the rectifying diode 43 and theabove-described second end of the secondary winding 32 at the connectionpoint P8.

In the rectifying and smoothing circuit 4A having such a configuration,as in the rectifying and smoothing circuit 4, the rectifying circuitincluding the rectifying diodes 41 to 44 may rectify thealternating-current voltage (the voltage Vs) outputted from thetransformer 3A, and then output the rectified voltage.

Workings and Effects

The switching power supply unit 1A of Modification Example 1 having sucha configuration is basically able to provide effects similar to those ofthe switching power supply unit 1 of the foregoing example embodimentthrough similar workings.

Further, in Modification Example 1, the rectifying circuit in therectifying and smoothing circuit 4A may be a bridge rectifying circuit,in particular. This reduces the number of the windings, that is, reducesthe number of the secondary windings to one (i.e., the secondary winding32) in the transformer 3A as compared with the foregoing exampleembodiment, for example. As a result, it is possible to achievereductions in size and loss of the transformer 3A.

Modification Examples 2 and 3

Switching power supply units according to Modification Examples 2 and 3,that is, switching power supply units 1B and 1C, respectively correspondto the switching power supply units 1 and 1A according to the foregoingexample embodiment and Modification Example 1 in which the rectifyingdiodes D1 and D2 in the respective inverter circuits 2 and rectifyingcircuits in the rectifying and smoothing circuits 4 and 4A are eachformed into a so-called synchronous rectifying circuit, as describedbelow.

Specifically, FIG. 13 illustrates a schematic configuration example ofthe switching power supply unit according to Modification Example 2(i.e., the switching power supply unit 1B) in a circuit diagram.

The switching power supply unit 1B of Modification Example 2 correspondsto the power supply unit 1 of the foregoing example embodiment in whichthe inverter circuit 2 and the rectifying and smoothing circuit 4 arereplaced with an inverter circuit 2B and a rectifying and smoothingcircuit 4B, respectively. The remainder of configuration of theswitching power supply unit 1B may be similar to that of the switchingpower supply unit 1.

In the synchronous rectifying circuit of Modification Example 2, asillustrated in FIG. 13 , the rectifying diodes D1 and D2 and therectifying diodes 41 and 42 described in relation to the exampleembodiment may be configured by MOS-FETs, i.e., MOS transistors M1 toM4, respectively. Further, the synchronous rectifying circuit iscontrolled to perform synchronous rectification, that is, to allow theMOS transistors M1 to M4 themselves to be turned on in synchronizationwith periods during which the respective parasitic diodes of the MOStransistors M1 to M4 are conducting. In a more specific but non-limitingexample, the driving circuit 5 of Modification Example 2 may beconfigured to control the on and off operations of the MOS transistorsM1 to M4 by using drive signals SG5 to SG8, respectively (see FIG. 13 ).

Further, the switching power supply unit according to ModificationExample 3 (i.e., the switching power supply unit 1C) corresponds to thepower supply unit 1A of Modification Example 1 in which the rectifyingdiodes D1 and D2 in the inverter circuit 2 and the rectifying diodes 41to 44 in the rectifying and smoothing circuit 4A are each configured bya MOS-FET. In the synchronous rectifying circuit of Modification Example3 also, as with the synchronous rectifying circuit of ModificationExample 2 described above, the driving circuit 5 performs control toallow the MOS-FETs themselves to be turned on in synchronization withthe periods during which the respective parasitic diodes of the MOS-FETsare conducting.

As with the foregoing example embodiment and Modification Example 1, asystem including the direct-current input power source 10 and theswitching power supply unit 1B or 1C may correspond to a specific butnon-limiting example of the “electric power supply system” of oneembodiment of the technology.

The switching power supply units 1B and 1C of Modification Examples 2and 3 having such configurations are basically able to provide effectssimilar to those of the switching power supply units 1 and 1A of theforegoing example embodiment and Modification Example 1 through similarworkings.

In each of Modification Examples 2 and 3, in particular, the two or morerectifying devices (rectifying diodes) in the rectifying circuit mayeach be configured by a MOS-FET, and the rectifying circuit may be asynchronous rectifying circuit. Such a synchronous rectifying circuitreduces a conduction loss upon rectification. Accordingly, it ispossible to achieve reductions in size and loss of the rectifyingcircuit.

3. Other Modification Examples

The technology has been described above with reference to the exampleembodiment and the modification examples. However, embodiments of thetechnology are not limited thereto, and may be modified in a variety ofways.

For example, in the foregoing example embodiment and the modificationexamples, description has been given of specific configurations of theinverter circuit by way of example. However, such examples arenon-limiting, and any other configuration may be employed for theinverter circuit. More specifically, for example, arrangement of theresonant capacitor Cr, the resonant inductor Lr, and the primary winding31 with respect to each other is not limited to the one described in anyof the example embodiment and the modification examples, and thesecomponents may be arranged in no particular order with respect to eachother.

Further, in the foregoing example embodiment and the modificationexamples, description has been given of specific configurations of thetransformer (the primary winding and the secondary winding) by way ofexample. However, such examples are non-limiting, and any otherconfiguration may be employed for the transformer (the primary windingand the secondary winding).

Further, in the foregoing example embodiment and the modificationexamples, description has been given of specific configurations of therectifying and smoothing circuit (the rectifying circuit and thesmoothing circuit) by way of example. However, such examples arenon-limiting, and any other configuration may be employed for therectifying and smoothing circuit (the rectifying circuit and thesmoothing circuit).

In addition, in the foregoing example embodiment and the modificationexamples, description has been given of specific techniques by which thedriving circuit controls (i.e., performs switching driving of) theoperations of the switching devices by way of example. However, suchexamples are non-limiting, and any other techniques may be employed asthe switching driving techniques. More specifically, for example, thetechnique for performing the pulse width control, the technique forforming the voltage Vp into a rectangular pulse wave, etc. described inthe foregoing example embodiment and the modification examples arenon-limiting, and any other techniques may be employed.

Further, in the foregoing example embodiment and the modificationexamples, description has been given of a DC-DC converter as an exampleof the switching power supply unit according to an embodiment of thetechnology. However, any embodiment of the technology is applicable toother kinds of switching power supply units such as AC-DC converters.

Moreover, any two or more of the configuration examples described so farmay be combined and applied in a desired manner.

The technology encompasses any possible combination of some or all ofthe various embodiments and the modifications described herein andincorporated herein.

It is possible to achieve at least the following configurations from theforegoing embodiments and modification examples of the technology.

(1)

-   -   A switching power supply unit including:    -   a pair of input terminals configured to receive an input        voltage;    -   a pair of output terminals configured to output an output        voltage;    -   a transformer including a primary winding and a secondary        winding;    -   an inverter circuit disposed between the pair of input terminals        and the primary winding, and including first to fourth switching        devices, first to third capacitors, first and second rectifying        devices, a resonant inductor, and a resonant capacitor;    -   a rectifying and smoothing circuit disposed between the pair of        output terminals and the secondary winding, and including a        rectifying circuit and a smoothing circuit, the rectifying        circuit including two or more rectifying devices, the smoothing        circuit including a fourth capacitor; and    -   a driver configured to perform switching driving to control        respective operations of the first to fourth switching devices        in the inverter circuit, in which    -   the first to fourth switching devices are coupled in series to        each other in this order between two input terminals        constituting the pair of input terminals,    -   the first capacitor and the second capacitor are coupled in        series to each other between the two input terminals        constituting the pair of input terminals,    -   the first rectifying device is disposed between a first        connection point and a second connection point, the first        connection point being a connection point between the first        capacitor and the second capacitor, the second connection point        being a connection point between the first switching device and        the second switching device,    -   the second rectifying device is disposed between the first        connection point and a third connection point, the third        connection point being a connection point between the third        switching device and the fourth switching device,    -   the third capacitor is disposed between the second connection        point and the third connection point, and    -   the resonant capacitor, the resonant inductor, and the primary        winding are coupled in series to each other in no particular        order between a fourth connection point and the first connection        point, the fourth connection point being a connection point        between the second switching device and the third switching        device.        (2)    -   The switching power supply unit according to (1), in which the        resonant inductor is configured by a leakage inductance of the        transformer.        (3)    -   The switching power supply unit according to (1) or (2), in        which each of the first to fourth switching devices is        configured by a metal oxide semiconductor-field effect        transistor.        (4)    -   The switching power supply unit according to any one of (1) to        (3), in which the rectifying circuit includes a center-tap        rectifying circuit.        (5)    -   The switching power supply unit according to any one of (1) to        (3), in which the rectifying circuit includes a bridge        rectifying circuit.        (6)    -   The switching power supply unit according to any one of (1) to        (5), in which    -   each of the two or more rectifying devices is configured by a        metal oxide semiconductor-field effect transistor, and    -   the rectifying circuit includes a synchronous rectifying        circuit.        (7)    -   The switching power supply unit according to any one of (1) to        (6), in which the driver is configured to perform the switching        driving in such a manner that respective switching frequencies        of the first to fourth switching devices are identical with each        other.        (8)    -   The switching power supply unit according to any one of (1) to        (7), in which the driver is configured to control a value of the        output voltage by adjusting a start timing or a stop timing of        each of respective periods during which the first to fourth        switching devices are to be on.        (9)    -   The switching power supply unit according to any one of (1) to        (8), in which    -   a switching cycle in the switching driving includes:        -   a first ON period during which both the first switching            device and the second switching device are to be on;        -   a second ON period during which both the third switching            device and the fourth switching device are to be on;        -   a first dead time during which both the first switching            device and the fourth switching device are to be off; and        -   a second dead time during which both the second switching            device and the third switching device are to be off, and    -   the driver is configured to control a value of the output        voltage by preventing the first ON period and the second ON        period from overlapping each other in the switching cycle and by        adjusting each of a duty ratio of the first ON period and a duty        ratio of the second ON period to the switching cycle.        (10)    -   The switching power supply unit according to any one of (1) to        (9), in which the driver is configured to control a value of the        output voltage by adjusting respective switching frequencies of        the first to fourth switching devices and respective periods        during which the first to fourth switching devices are to be on.        (11)    -   An electric power supply system including:    -   the switching power supply unit according to any one of (1) to        (10); and    -   a power source configured to supply the input voltage to the        pair of input terminals.

The switching power supply unit and the electric power supply systemaccording to at least one embodiment of the technology make it possibleto reduce a power loss.

Although the technology has been described hereinabove in terms of theexample embodiment and modification examples, it is not limited thereto.It should be appreciated that variations may be made in the describedexample embodiment and modification examples by those skilled in the artwithout departing from the scope of the disclosure as defined by thefollowing claims. The limitations in the claims are to be interpretedbroadly based on the language employed in the claims and not limited toexamples described in this specification or during the prosecution ofthe application, and the examples are to be construed as non-exclusive.The use of the terms first, second, etc. do not denote any order orimportance, but rather the terms first, second, etc. are used todistinguish one element from another. The term “substantially” and itsvariants are defined as being largely but not necessarily wholly what isspecified as understood by one of ordinary skill in the art. The term“disposed on/provided on/formed on” and its variants as used hereinrefer to elements disposed directly in contact with each other orindirectly by having intervening structures therebetween. Moreover, noelement or component in this disclosure is intended to be dedicated tothe public regardless of whether the element or component is explicitlyrecited in the following claims.

What is claimed is:
 1. A switching power supply unit comprising: a pairof input terminals configured to receive an input voltage; a pair ofoutput terminals configured to output an output voltage; a transformerincluding a primary winding and a secondary winding; an inverter circuitdisposed between the pair of input terminals and the primary winding,and including: first to fourth switching devices coupled in series toeach other in that order between the pair of input terminals; first tothird capacitors, the first capacitor and the second capacitor beingcoupled in series to each other between the pair of input terminals;first and second rectifying devices; a resonant inductor; and a resonantcapacitor; a rectifying and smoothing circuit disposed between the pairof output terminals and the secondary winding, and including: arectifying circuit including: a first rectifying device disposed betweena first connection point and a second connection point, the firstconnection point being between the first capacitor and the secondcapacitor, the second connection point being between the first switchingdevice and the second switching device; and a second rectifying devicedisposed between the first connection point and a third connection pointbetween the third switching device and the fourth switching device; anda smoothing circuit including a fourth capacitor; and a driverconfigured to perform switching driving to control respective operationsof the first to fourth switching devices in the inverter circuit suchthat a switching cycle in the switching driving includes: a first ONperiod during which both the first switching device and the secondswitching device are to be on; a second ON period during which both thethird switching device and the fourth switching device are to be on; afirst dead time during which both the first switching device and thefourth switching device are to be off; and a second dead time duringwhich both the second switching device and the third switching deviceare to be off, the driver being configured to control a value of theoutput voltage by preventing the first ON period and the second ONperiod from overlapping each other in the switching cycle and byadjusting each of a duty ratio of the first ON period and a duty ratioof the second ON period to the switching cycle, wherein the thirdcapacitor is disposed between the second connection point and the thirdconnection point, and the resonant capacitor, the resonant inductor, andthe primary winding are coupled in series to each other between a fourthconnection point and the first connection point, the fourth connectionpoint being a connection point between the second switching device andthe third switching device.
 2. The switching power supply unit accordingto claim 1, wherein the resonant inductor is a leakage inductance of thetransformer.
 3. The switching power supply unit according to claim 1,wherein each of the first to fourth switching devices is a metal oxidesemiconductor-field effect transistor.
 4. The switching power supplyunit according to claim 1, wherein the rectifying circuit comprises acenter-tap rectifying circuit.
 5. The switching power supply unitaccording to claim 1, wherein the rectifying circuit comprises a bridgerectifying circuit.
 6. The switching power supply unit according toclaim 1, wherein each of the two or more rectifying devices is a metaloxide semiconductor-field effect transistor, and the rectifying circuitcomprises a synchronous rectifying circuit.
 7. The switching powersupply unit according to claim 1, wherein the driver is configured toperform the switching driving in such a manner that respective switchingfrequencies of the first to fourth switching devices are identical witheach other.
 8. The switching power supply unit according to claim 1,wherein the driver is configured to control the value of the outputvoltage by adjusting a start timing or a stop timing of each ofrespective periods during which the first to fourth switching devicesare to be on.
 9. The switching power supply unit according to claim 1,wherein the driver is configured to control the value of the outputvoltage by adjusting respective switching frequencies of the first tofourth switching devices and respective periods during which the firstto fourth switching devices are to be on.
 10. An electric power supplysystem comprising: the switching power supply unit according to claim 1;and a power source configured to supply the input voltage to the pair ofinput terminals.